Hadi Bux

Expression of high-temperature adult-plant (HTAP) resistance against stripe rust (Puccinia striiformis f. sp. tritici) in Pakistan wheat landraces

Abstract Stripe rust (Puccinia striiformis f. sp. tritici, Pst) is an important disease of wheat in the United States and Pakistan. Genetic resistance in wheat is a cost-effective and convenient control measure. In the present study, resistance testing of 115 wheat landraces from Pakistan was carried out initially at the seedling stage with seven races of the pathogen, four from the USA and three from Pakistan. The germplasm showed a general lack of effective seedling resistance. Only two genotypes (PI 210916, PI 219752) were resistant against all races. Adult plants of the wheat genotypes were also tested at high temperatures under greenhouse conditions and at two field locations in Washington State. In the test with US Pst race PST-127, 28 (32%) of the landraces had seedling resistance and 41 (52%) had high-temperature adult-plant (HTAP) resistance. When tested with Pakistan isolate PK07-2, 14 (13%) of the wheat genotypes had seedling resistance and 61 (53%) had HTAP resistance. In the test with Pakistan isolate PK07-6, 32 (40%) had seedling resistance and 38 (48%) had HTAP resistance. In field tests, 94 (86%) genotypes were resistant and four (4%) were intermediate at Pullman, Washington; and 72 (89%) were resistant and none was intermediate at Mount Vernon, Washington. The greenhouse and field tests revealed that the majority of the wheat landraces possess HTAP resistance against stripe rust. These landraces should be useful genetic resources for developing stripe rust resistant cultivars in regions where climatic conditions are suitable for the expression of the durable type of resistance.

An overview of stripe rust of wheat (Puccinia striiformis f. sp. tritici) in Pakistan

Stripe rust (yellow rust) caused by Puccinia striiformis f. sp. tritici has been an important disease of wheat in the Indian subcontinent since 1786. Currently, it prevails across all the wheat growing areas from north to south in the country. Due to the favourable weather conditions, the northern uplands have been historically hit by the severe disease epidemics. These epidemics caused significant losses to national wheat production. Acquisition of broader virulence pattern by the pathogen poses a serious threat to national agriculture. Although the deployed national wheat varieties have adequate resistance, these are developed around few major genes and are vulnerable to the new evolving strains of the pathogen. Utilisation of race non-specific durable resistance and seedling resistance via gene pyramiding, based on the current virulence scenario of the pathogen should provide sustainable control. This review focuses on the national milestones that recognise the economic significance of the disease and current status of stripe rust and its management in Pakistan.

Powdery mildew resistance in some new wheat amphiploids (2n 5 6x 5 42) derived from A- and S-genome diploid progenitors

Triticum urartu possesses the A u genome common to bread wheat. Similarly, Triticum monococcum contains the A m genome, which is closely related to the A-genome donor of bread wheat. Aegilops speltoides of the Sitopsis section has the S genome, which is most similar to the B genome of bread and durum wheat when compared with all other wild grasses. Amphiploids developed through bridge crossing between A m /A u and S-genome diploid resources and elite durum cultivars demonstrate enormous diversity to improve both bread and durum wheat cultivars. We evaluated such A-genome amphiploids (Triticum turgidum £ T. urartu and T. turgidum £ T. monococcum ,2 n ¼ 6x ¼ 42; BBAAA m A m /A u A u ) and S-genome amphiploids (T. turgidum £ Ae. speltoides ,2 n ¼ 6x ¼ 42; AABBSS) along with their durum parents (AABB) for their resistance to powdery mildew (PM) at the seedling stage. The results indicated that 104 accessions (53.6%) of A-genome amphiploids (AABBA m A m /A u A u ) were resistant to PM at the seedling stage. Of their 24 durum parents, five (20.83%) were resistant to PM and 16 (66.6%) were moderately tolerant. Similarly, ten (50%) accessions of S-genome amphiploids (BBAASS) possessed seedling PM resistance, suggesting a valuable source of major resistance genes. PM screening of the amphiploids and parental durum lines showed that resistance was contributed either by the diploid progenitors or durum parents, or both. We also observed the suppression of resistance in several cases; for example, resistance in durum wheat was suppressed in respective amphiploids. The results from this germplasm screening will facilitate their utilization to genetically control PM and widen the genetic base of wheat.

Molecular Basis of Disease Resistance in Cereal Crops: An Overview

Deep insight into the molecular mechanism of disease resistance in plants is essential for devising sophisticated breeding strategies leading towards crop protection. Most of the disease resistance (R) genes show genetic and structural similarity even though their target pathogens are extremely diverged in strain types, mode of action, virulence and target plant part. The search for a unified model operating for disease resistance in cereals may provide improved control of devastating pathogens which is critical for global food security. A super family of ‘R’ genes encoding NBS-LRR (Nucleotide binding site- Leucine rich repeats) with its several sub-classes, recently identified ABC (ATP binding cassette) transporter and kinase START genes confer resistance to numerous pathogens. These molecular motifs activate a wide array of metabolic responses under tight genetic control as the plant detects a prospective invader. Some of the ‘R’ genes require additional genes for the expression of resistance. Investigations into the structure of ‘R’ genes, protein products, their location and mechanism of interaction with pathogen elicitor molecules and future prospects are discussed in this review.

Stripe rust analysis of D-genome synthetic wheats (2n = 6x = 42, AABBDD) and their molecular diversity

Stripe or yellow rust caused by Puccinia striiformis f. sp. tritici is a threat to many of the existing cultivars of Pakistan. Many attempts are being made to evolve new varieties resistant to stripe rust to reduce the losses caused by this disease. For this purpose, novel genes are needed to incorporate into the existing cultivars. These genes are found in the wild progenitors of wheat that are D-genome donors to wheat. As a result of extensive research, wheat synthetic hexaploids have been developed. These synthetics have resistances against biotic as well as abiotic stresses including the yellow rust. A group of such synthetics has been identified which seems resistant to this destructive disease. This group was tested under field conditions to identify resistance against stripe rust. The same population was analysed at molecular level to explore the genetic diversity for rust resistance. Genetic diversity among 34 selected synthetic hexaploid wheats was studied by random amplified polymorphic DNA (RAPD) analysis. A set of 12 RAPD primers was applied, and the level of polymorphism was found to be 46.67%. The coefficients in the range of 71–100% were detected by genetic similarity matrix based on Nei and Lis index. These coefficients were used for constructing a dendrogram using unweighted pair group of arithmetic means. Synthetic hexaploid line 34 was found to exhibit maximum genetic distances among the 34 selected lines. The same accession also showed excellent phenotypic characters with above average grain weight. These synthetic hexaploids carrying genetic potential for stripe rust resistance and morphological traits should be useful for improvement of existing wheat cultivars.

Cytological, Phenological and Molecular Characterization of B (S)-Genome Synthetic Hexaploids (2n = 6x = 42; AABBSS)

The B(S) genome diploids (2n = 2x = 14) are a unique reservoir of genetic diversity that can provide wheat breeders a rich source of allelic variation for stress traits that limit productivity. Restricted in practical use essentially due to their complex chromosomal behavior, these diploids have been in limited practical usage. The classic utilization example has been the suppression activity of the Ph locus and role in alien genetic transfer aspects that has been a standard in cytogenetic manipulation studies. For applied efforts focusing on Aegilops speltoides researchers in CIMMYT initiated an ambitious program to make AABBBB(SS) synthetics and made progress by generating over 50 such synthetics. Of these 20 were available for this study in which phenology and powdery mildew screening were evaluated. Four of these 20 synthetics appeared to be useful sources for further exploitation in breeding. These were entries 6, 9, 10 and 11 suited for exploitation in pre-breeding, with positive phenological characters particularly high thousand-kernel weight and are cytologically near euploid at 2n = 6x = 42. The subtle hyper (43) and hypoploid number would not negate their applied use potential. Preference however goes to genotypes 9 and 11.

A conspectus of Barley Yellow Dwarf in Pakistan

Wheat is an important cereal crop worldwide and in Pakistan. Among the wheat producing countries, Pakistan ranks 6th with an annual wheat production of 24.214 million tons. Burgeoning population of the country demands increase in its production that is hindered by a number of pests, pathogens and environmental stresses. Among the yield limiting constraints, Barley Yellow Dwarf Virus (BYDV) is important, inflicting approximately 75% wheat production losses in diseased crop nationally. Research on BYDV and its management is mandatory for sustainable agriculture in the country. This review focuses on BYDV, its relationship with vectors, classification and management practices. Additionally, its currents status in Pakistan has been reviewed.